The invention contemplates an engine fuel injector comprising an internal needle valve having a tip that opens or closes a flow path leading to the combustion space. The other end of the needle valve communicates with a control chamber that at times is pressurized by the fuel pressure source; control chamber pressure exerts an axial force on the needle that opposes the force on the needle provided by the fuel in the main flow path.
A spring is arranged to bias the needle valve to a closed position when both the main flow path and control chamber are pressurized to the maximum flow. When the control chamber is depressurized the spring is unable to hold the needle valve closed; fuel pressures at the tip area of the needle move it to the open condition. By alternately pressurizing or depressurizing the control chamber it is possible to rapidly close or open the flow path leading to the combustion space.
The "balancing" action produced by the pressurized control chamber enables the fuel flow path to be highly pressurized without premature opening of the needle valve. Therefore the initial flow of fuel into the combustion space is at high pressure, with consequent advantages relative to finer atomization of the fuel. The biasing spring establishes a low pressure cut-off that prevents unduly low injection pressure near the end of the injection period.
In a preferred embodiment of the invention the fuel flow path comprises a pressurizable fuel storage chamber. After the initial pressurization of the storage chamber volume by completion of the pump delivery stroke, the pressurized chamber is automatically isolated from the fuel pressure source (pump); the pump then begins its suction stroke, and a check valve confines the retained volume under pressure for injection into the cylinder. As fuel is ejected from the storage chamber the fuel pressure at the discharge orifice is reduced so that the valve-closing spring is able to produce a rapid shut-off action without undue opposition from the flowing fuel.